The 60 GHz band is an unlicensed band which features a large amount of bandwidth and a large worldwide overlap. The large bandwidth means that a very high volume of information can be transmitted wirelessly. As a result, multiple applications that require transmission of a large amount of data can be developed to allow wireless communication around the 60 GHz band. Examples of such applications include, but are not limited to, wireless high definition TV (HDTV), wireless docking stations, wireless Gigabit Ethernet, and many others. Wireless local area network (WLAN) standards, such as WiGig Alliance (WGA) and IEEE 802.11ad, are being developed to serve applications that utilize the 60 GHz spectrum.
Such communication standards enable wireless transmission between two stations that are a short distance from each other. Typically, in such wireless transmission systems, signals circulate between transmitters and receivers by way of channels. Due to many factors in a channel's characteristics, an unwanted distortion may be induced in the signal transmitted by the transmitter. Accordingly, it is generally necessary to determine the characteristics of a channel at a given moment in order to estimate the induced distortion in the transmitted signal.
Signal acquisition is required to enable proper operation of the wireless system. Preferably, signal acquisition should be performed in any type of channel condition as defined by for the operation of wireless transmission system. In particular, in a millimeter-wave wireless transmission system operating in the 60 GHz band, for example, as defined by the IEEE 802.11ad standard published Dec. 28, 2012 (hereinafter the IEEE 802.11ad standard), an initial signal acquisition includes periodically identifying detection sequences in the received signal and then trying to synchronize the sequences to a starting time of frames carrying the transmitted data.
Typically, a transmitted frame includes a preamble portion and a payload portion. The preamble is used for signal and packet detection, AGC setting, frequency offset estimation, timing synchronization, indication of antenna diversity selection and modulation (OFDM or SC), and channel estimation. The format of the preamble is common to both OFDM packets and SC packets.
As shown in FIG. 1, an IEEE 802.11ad preamble 100 of a PLOP protocol data unit (PPDU) packet is composed of two parts, a short training field (STF) 120 and a channel estimation field (CEF) 130. Both the STF 120 and CEF 130 fields contain Golay complementary sequences, which are transmitted by a transmitter and are correlated by a receiver in a millimeter-wave wireless transmission system. Typical Golay complementary sequences have many advantageous properties, such as producing a perfect sum of correlations and providing efficient implementations requiring only log2(N) additions for two complementary sequences of length N.
The STF 120 is composed of 16 repetitions of the sequence Ga128(n) in length 128 followed by a single repetition of −Ga128(n). The CEF 130 is used for channel estimation, as well as for indicating which modulation is going to be used for the packet. The CEF 130 is composed of a concatenation of two sequences, Gu512(n) and Gv512(n), where the last 128 samples of both Gu512(n) and Gv512(n) are equal to the last 128 samples used in the short training field (e.g., −Ga128(n)). These sequences are followed by a 128 sample sequence, Gv128(n), equal to the first 128 samples of both Gu512(n) and Gv512(n).
There are a number of techniques for initial signal acquisition in millimeter-wave wireless transmission systems. Typically, an initial signal acquisition requires detection and timing synchronization. One such technique for initial signal acquisition is based on an energy detector which is relatively simple to design. The signal is detected by comparing the output of the energy detector with a threshold defined by the noise energy. The noise level can only be estimated with limited accuracy due, for example, to the fact that the antenna noise varies as a function of indoor and/or outdoor conditions.
The noise induced by the communication channel and/or wireless system may vary over frequency and during operations, and as such the noise level estimation itself always has some error. Therefore, a detection threshold level determined in part by the estimated noise has no means to differentiate between the actual signal and induced noise as well as to distinguish between signals transmitted by transmitters in the vicinity of the receiver.
Another technique for signal acquisition and particularly for signal detection, as defined in the IEEE 802.11ad standard, is based on an auto-correlation of the received signal within a periodic detection sequence length. The main advantage of this technique is immunity to multipath channels and frequency offset below an ambiguity limit. This technique also allows the auto-correlation to be averaged in order to improve SNR sensitivity performance of the detection flow. However, auto-correlation of the received raw signals, i.e., signals without any data-processing, may cause false alarm detections in the presence of any periodic interfering signals (e.g.—DC or LO leakage).
Another technique for signal detection is based on, in part, cross-correlation. The cross-correlation of the received signal is typically performed with an IEEE 802.11ad pre-defined detection-sequence pattern. The main advantage of this technique is that the technique is robust against gain variations and other analog imperfections, hence its improved performance for lower SNRs relative to other techniques.
An initial signal acquisition process further requires a signal timing synchronization to detect a starting of the frame. In the IEEE 802.1 lad standard, for example, the signal timing synchronization is detected using cross-correlation of a start frame delimiter (SFD). As noted above, the IEEE 802.11ad preambles contain several repetitions of Golay-128 pattern, followed by the SFD-Channel-Estimation Golay-512-patterns.
The disadvantage of the correction techniques is that a threshold for comparison of the correlation results is either constant, or relative to the energy level of the received signals. Typically, the detection-threshold causes an un-trustable detection performance in low SNR ranges in the presence of a multipath channel.
It would be therefore advantageous to provide a solution that would overcome the disadvantages of existing techniques for signal acquisition in millimeter-wave wireless systems.